Process for producing refined metal or metalloid

ABSTRACT

Process for producing a refined metal or metalloid, comprising: an electrolysis step of, in an electrolytic bath set in a container for an electrolysis in which a material comprising a metal element or metalloid element and impurities acts as an anode, and an alloy comprising the same metal element or metalloid element as the metal element or metalloid element and a medium metal that does not substantially form a solid solution with the metal element or metalloid element and having a complete solidification temperature lower than the melting point of the metal element or metalloid element acts as a cathode, performing electrolysis at a temperature at which the alloy can be a liquid phase; withdrawing a part or the whole of the alloy of the cathode to an outside of the container; cooling the withdrawn alloy at a temperature higher than the complete solidification temperature and lower than the electrolysis temperature; and a recovery step.

TECHNICAL FIELD

The present invention relates to process for producing a refined metal or metalloid.

BACKGROUND ART

Metallurgical grade silicon is produced by mixing carbon with silica and performing reduction in an arc furnace. Trichlorosilane is synthesized by the reaction of the metallurgical grade silicon with HCl, and refined by rectification; subsequently, reduction is performed at a high temperature using hydrogen; thus, semiconductor grade silicon is produced. At present, as a principal raw material for silicon used for solar cells, an off-grade product produced during the manufacturing of the semiconductor grade silicon is used.

In the process for producing the semiconductor grade silicon, silicon with extremely high purity can be produced, but the process is expensive for the following reasons: the conversion rate from trichlorosilane gas to silicon is low and a large amount of hydrogen is needed to make equilibrium to silicon advantageous; a large amount of non-reacted gas needs to be circulated and reused in order to increase the conversion ratio; a variety of silane halides coexist in the non-reacted trichlorosilane gas, and therefore separation by rectification is needed again; a large amount of silicon tetrachloride that cannot be reduced with hydrogen finally is produced, and the like.

On the other hand, solar cells receive attention as a potent solution for environmental issues such as increase in carbon dioxide gas these days, and the demand for the solar cells has been remarkably increased. For this reason, if only the off-grade product of the semiconductor grade silicon is used for the raw material for the solar cells, shortage of the raw material in future may be caused. Moreover, because silicon for solar cells is expensive and the like, the solar cells at present are still expensive. The price of electricity obtained by the solar cell is several times that of the commercial electricity, and supply of inexpensive silicon for solar cells has been desired.

Silicon can be also refined by electrolysis; in Patent Literature 1 and Patent Literature 2, electrolysis refinement using solid silicon as a cathode is examined; in Patent Literature 3, electrolysis refinement using fused silicon as a cathode is examined.

CITATION LIST Patent Literature

-   Patent Literature 1: WO2008/115072 -   Patent Literature 2: U.S. Pat. No. 3,219,561 -   Patent Literature 3: U.S. Pat. No. 3,254,010

SUMMARY OF INVENTION Technical Problem

In the electro-refining process of silicon, by the processes disclosed in Patent Literature 1 and Patent Literature 2 in which electrolysis is performed using solid silicon as the cathode, silicon deposited on the cathode grows dendritically to cause a short circuit between electrodes; for this reason, it is difficult to continue electrolysis, and it is remarkably difficult to prevent an electrolytic bath from being trapped in a deposit. Moreover, in the process disclosed in Patent Literature 3 in which electrolysis is performed using fused silicon as the cathode, if the electrolysis temperature reaches approximately 1410° C. or more that is the melting point of silicon, the reverse reaction of reduced silicon is made to reduce the current efficiency of the electrolysis, and choice of suitable furnace materials that can be used at high temperature is limited; for such reasons, industrialization of the process is difficult. For the same reason, with respect to electro-refining of materials of metals and metalloids other than silicon such as germanium, particularly materials whose melting point is relatively high or that are difficult to be electrolyzed in an aqueous solution, industrialization is difficult in most cases.

An object of the present invention is to provide a process for producing a refined metal or metalloid in which an electrolysis temperature can be lower than the melting point of a metal element or metalloid element that is an object to be refined, and the dendritic growth of the refined product and the trapping of an electrolytic bath in the refined product can be suppressed.

Solution to Problem

A process for producing a refined metal or metalloid according to the present invention comprises an electrolysis step, a withdrawal step, a deposition step, and a recovery step.

In the electrolysis step, in an electrolytic bath set in a container for an electrolysis in which a material comprising a metal element or metalloid element and impurities is subjected to act as an anode, and an alloy comprising the same metal element or metalloid element as the metal element or metalloid element contained in the anode and a medium metal that does not substantially form a solid solution with the metal element or metalloid element and having a complete solidification temperature lower than the melting point of the metal element or non-metal element is subjected to act as a cathode, electrolysis is performed at an electrolysis temperature at which the alloy of the cathode can be a liquid phase to move the metal element or metalloid element in the anode to the alloy of the cathode.

The complete solidification temperature of an alloy is a temperature corresponding to the lowest value of a liquidus in a solid-liquid phase diagram of the alloy; at a temperature less than the complete solidification temperature, the alloy cannot contain a liquid phase.

That the medium metal that does not substantially form a solid solution with the metal element or metalloid element means that the solid solubility limit of the medium metal to the metal element or metalloid element at the complete solidification temperature is not more than 1% by mass.

In the withdrawal step, after the electrolysis step, a part or the whole of the alloy of the cathode is withdrawn to an outside of the container for the electrolysis. In the withdrawal step, the alloy of the cathode in which the concentration of the metalloid element or metal element is higher than that of the composition corresponding to the complete solidification temperature of the alloy may be withdrawn to an outside of the container for the electrolysis.

In the deposition step, the withdrawn alloy is cooled at a temperature higher than the complete solidification temperature and lower than the electrolysis temperature to deposit the metal element or metalloid element contained in the alloy.

In the recovery step, the metal element or metalloid element deposited from the cooled alloy is recovered.

According to the present invention, because the alloy comprising the metal element or metalloid element and the medium metal and having the complete solidification temperature lower than the melting point of the metal element or metalloid element is subjected to act as the cathode, the electrolysis temperature needed to make the cathode a liquid phase can be reduced compared to the case where a single metal element or metalloid element is subjected to act as the cathode. Moreover, because in the electrolysis step, the cathode is a liquid phase, a short circuit between electrodes caused by dendritic growth of the metal element or metalloid element and trapping of the electrolytic bath in the refined product of the element or metalloid element can be suppressed.

Because the medium metal does not substantially form the solid solution with the metal element or metalloid element, the concentration of the metal element or metalloid element in the alloy of the cathode subjected to the electrolysis step and the withdrawal step and brought to the deposition step is higher than that of the composition corresponding to the complete solidification temperature; thereby, the metal element or metalloid element contained in the alloy can be selectively deposited with high purity by cooling. Thereby, the purity of the recovered metal element or metalloid element can be sufficiently higher than that of the material that forms the anode, and the refined metal element or metalloid element can be easily obtained.

The medium metal may have a eutectic point with the metal element or metalloid element. In this case, the concentration of the metal element or metalloid element in the alloy of the cathode subjected to the electrolysis step and the withdrawal step and brought to the deposition step is higher than that of the composition corresponding to the eutectic point; thereby, the metal element or metalloid element contained in the alloy can be selectively deposited with further higher purity by cooling.

Here, it is preferable that the process further comprise a reuse step of using a residue after the metal element or metalloid element deposited is recovered from the cooled alloy of the cathode, as the cathode in the electrolysis step. The residue means a remaining object, and it may be liquid or solid.

Thereby, the residue in which the concentration of the metal element or metalloid element is sufficiently reduced is reused as the cathode in the electrolysis step; accordingly, movement of the metal element or metalloid element from the anode to the cathode can be continuously performed with efficiency.

It is preferable that the metal element or metalloid element be silicon or germanium.

In the case where the metal element or metalloid element is silicon or germanium, it is preferable that the alloy of the cathode comprise one or more metal elements selected from the group consisting of aluminum, silver, copper, and zinc.

It is preferable that the material for the anode comprise one or more metal elements selected from the group consisting of silver, copper, tin, and lead.

It is preferable that the electrolytic bath comprise cryolite.

It is preferable that the electrolysis temperature be higher than the complete solidification temperature and lower than the melting point of the metal element or metalloid element.

Advantageous Effects of Invention

According to the present invention, a process for producing a refined metal element or metalloid element in which an electrolysis temperature can be lower than the melting point of the metal element or metalloid element that is an object to be refined, and dendritic growth of a refined product and trapping of an electrolytic bath in the refined product can be suppressed is provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a solid-liquid phase diagram of a germanium-lead system.

FIG. 2 is a solid-liquid phase diagram of a silicon-aluminum system.

DESCRIPTION OF EMBODIMENTS

The process for producing a refined metal or metalloid according to the present invention mainly comprises an electrolysis step, an withdrawal step, a deposition step, a recovery step, and when necessary, a reuse step.

(Metal Element or Metalloid Element)

In the present invention, a metal element or metalloid element (hereinafter, referred to as an element to be refined in some cases) is an object to be refined. The element to be refined is not particularly limited. Examples of the metal element include alkaline earth metals such as beryllium; first transition elements such as scandium, titanium, and nickel; second transition elements such as zirconium and yttrium; lanthanoids such as lanthanum, neodymium, europium, dysprosium, rhenium, and samarium; actinoids such as thorium, uranium, plutonium, and americium; and third transition elements such as platinum.

Examples of the metalloid element include silicon, arsenic, antimony, and germanium.

Among these metal elements or metalloid elements, silicon, germanium, nickel, lanthanoids, and actinoids are preferable, and silicon and germanium are particularly preferable, considering easiness in recovering from the alloy of the cathode that is a liquid phase.

(Electrolysis Step)

In the electrolysis step, in an electrolytic bath set in a container for an electrolysis in which a material comprising an element to be refined and impurities is subjected to act as an anode, and an alloy comprising the same element to be refined as the element to be refined contained in the anode and a medium metal and having a complete solidification temperature lower than the melting point of the element to be refined (specifically, described later) is subjected to act as a cathode, electrolysis is performed at an electrolysis temperature at which the alloy of the cathode can be a liquid phase, thereby to move the element to be refined in the anode to the alloy of the cathode; thus, an alloy in which the concentration of the element to be refined is higher than the concentration of the element to be refined in the alloy composition corresponding to the complete solidification temperature (specifically, described later) is obtained on the cathode.

(Anode)

The material for the anode is a material comprising an element to be refined and impurities, and has another aspect as a raw material for refinement. The material for the anode may be those that are a solid phase at the electrolysis temperature; it is preferable for easiness in the electrolysis reaction that the material for the anode be those that are a liquid phase at the electrolysis temperature.

The impurities contained in the material for the anode are elements more noble than the element to be refined or elements less noble than the element to be refined. In the case where the element to be refined is silicon, examples of elements more noble than silicon include silver and copper, and examples of elements less noble than silicon include sodium and magnesium. The case of germanium is similar to the case of silicon; examples of elements more noble than germanium include silver and copper, and examples of elements less noble than geimanium include sodium and magnesium. The concentration of the impurities is not particularly limited, and the concentration is several tens ppm to several % based on the material for the anode in a mass ratio, for example.

It is preferable that the material for the anode be an alloy of the element to be refined and impurities different from the element to be refined (hereinafter, referred to as an anode medium metal in some cases), and it is more preferable that the material for the anode be an alloy having a eutectic point lower than the melting point of the element to be refined. In this case, it is preferable that in the alloy, the vapor pressure be low and stable. Moreover, it is preferable that the anode medium metal be an element more noble than the element to be refined. The element to be refined and anode medium metal can be properly selected on the basis of the theoretical decomposition voltage according to thermodynamic data, for example. Examples of the theoretical decomposition voltage of each element are shown below. Calculation of the theoretical decomposition voltage may be performed according to a method in which the dissolved species of each element is determined and the free energy to be produced is examined, or a method in which estimation is performed on the basis of the free energy to be produced of a metal compound such as a halide that is relatively easily available. For example, if the theoretical decomposition voltage in a fluoride fused salt is estimated on the basis of the free energy to be produced of each metal fluoride, the theoretical decomposition voltage is calculated as follows: 1.9 V for Cu, 2.8 V for Fe(II), 3.4 V for Ti(IV), 3.6 V for Mn(II), 3.7 V for Si, 4.1 V for Al, 4.6 V for K, 4.6 V for Na, 4.7 V for Mg, and 5.3 V for Ca. In the case of an element M other than Si, these theoretical decomposition voltages are calculated on an assumption that the reaction represented by the following formula progresses. The activity is 1 in each case, and the temperature is 1000° C.

MF_(x)(solid)−>M(solid)+x/2F₂(gas)

In the case of Si, the theoretical decomposition voltage is calculated on an assumption that the reaction represented by the following formula progresses. The activity is 1 in each case, and the temperature is 1000° C.

SiF₄(gas)−>Si(solid)+2F₂(gas)

In the case where the element to be refined is silicon, examples of the anode medium metal include one or more elements selected from the group consisting of copper, tin, silver, gold, mercury, and lead, and considering cost and an influence on the environment, preferable are one or more elements selected from copper, silver, tin, and lead. The alloy of the anode may contain two or more anode medium metals.

Moreover, as the purity of the anode medium metal, preferable is not less than 3 N, more preferable is not less than 5 N, and particularly preferable is not less than 6 N. The metals sufficiently more noble than silicon such as silver and copper and metals sufficiently less noble than silicon such as sodium and magnesium are removed by electrolysis; moreover, the metal used as the cathode medium metal, which will be described later, is removed in the deposition step and does not influence the purity of silicon; accordingly, the metal does not need to be considered as the impurities of the anode medium metal.

(Cathode)

As the cathode, used is an alloy comprising the same element to be refined as the element to be refined contained in the anode and a medium metal different from the element to be refined (hereinafter, referred to as a cathode medium metal in some cases) and having a complete solidification temperature lower than the melting point of the element to be refined. The cathode medium metal does not substantially form a solid solution with the element to be refined.

The complete solidification temperature of an alloy is a temperature corresponding to the lowest value of a liquidus in the solid-liquid phase diagram of the alloy; at a temperature less than the complete solidification temperature, the alloy cannot contain a liquid phase.

That the cathode medium metal does not substantially form a solid solution with the element to be refined means that the solid solubility limit of the cathode medium metal to the element to be refined at the complete solidification temperature is not more than 1% by mass.

The cathode medium metal may have a eutectic point with the element to be refined. Namely, an alloy of the cathode medium metal and the metal to be refined may have the minimum value in the liquidus.

Moreover, it is preferable that in the alloy, vapor pressure be low and stable.

In the case where the element to be refined is silicon, examples of such a cathode medium metal include one or more members selected from the group consisting of aluminum, copper, tin, gallium, indium, silver, gold, mercury, and lead; among them, considering cost and an influence on the environment, preferable are one or more members selected from the group consisting of aluminum, silver, copper, and zinc. The alloy may contain two or more cathode medium metals.

For example, a solid-liquid phase diagram of a germanium-lead system in which the element to be refined is Ge and the cathode medium metal is Pb is as shown in FIG. 1, and the complete solidification temperature in this system is 327° C. that is the lowest value in the liquidus A, i.e., a point B. The concentration of Ge (element to be refined) corresponding to the complete solidification temperature in the alloy is 0 wt %.

Moreover, a solid-liquid phase diagram of a silicon-aluminum system in which the element to be refined is Si and the cathode medium metal is Al is as shown in FIG. 2. The complete solidification temperature in this system is 577° C. that is a point C showing the lowest value (relative minimum value) in the liquidus A; as the point C, a point at which the liquidus has the minimum value is referred to as a eutectic point. The concentration of Si in the composition corresponding to the complete solidification temperature of the alloy is approximately 12.6 wt %, and the composition is a composition corresponding to the eutectic point and also referred to as a eutectic composition. The alloy system such as a silicon-silver system and a germanium-zinc system shows a phase diagram having the same eutectic point as that in FIG. 2.

Moreover, as the purity of the cathode medium metal, preferable is not less than 3 N, more preferable is not less than 5 N, and particularly preferable is not less than 6 N. Moreover, particularly, the contents of P and B each are preferably not more than 0.5 ppm, more preferably not more than 0.3 ppm, and particularly preferably not more than 0.1 ppm based on the cathode medium metal.

The ratio of the element in the alloy of the cathode at the time of starting electrolysis is not particularly limited, and a cathode medium metal containing no element to be refined (containing the alloy) may be used. However, when the withdrawal step is performed after the electrolysis step is performed, the concentration of the element to be refined in the alloy of the cathode needs to be more than that of the composition corresponding to the complete solidification temperature.

In the alloy having a eutectic point, the concentration of the element to be refined at the time of withdrawal needs to be higher than that of the composition corresponding to the eutectic point. In other words, for example, in FIG. 2, the concentration of Si that is the element to be refined in the alloy at the time of withdrawal needs to be more than 12.6 wt % that is the concentration of the eutectic composition. Moreover, particularly, in order to recover the element to be refined efficiently, it is preferable that at a determined electrolysis temperature the concentration of the element to be refined in the alloy of the cathode be increased by electrolysis until it is close to the saturated concentration of the element to be refined that is the maximum concentration of the element to be refined at which the alloy of the cathode can exist as a single phase of a liquid phase.

(Electrolytic Bath)

The electrolytic bath is not particularly limited as long as it is those that can conduct ions of the element to be refined, and metal halides are preferable. Examples of a metal element that forms a metal halide include one or more elements selected from alkali metals, alkaline earth metals, aluminum, zinc, and copper. Moreover, examples of a halogen that forms a metal halide include one or more element selected from the group consisting of fluorine, chlorine, and bromine. Moreover, two or more of these metal halides may be mixed and used. Examples of a mixture of metal halides include a mixture of sodium fluoride and aluminium fluoride. As the electrolytic bath, more specifically, preferable are cryolite (3NaF.AlF₃), calcium chloride, and the like for industrial availability. These electrolytic baths are used at fused state.

In the case where it is desired to increase the purity of the element to be refined as an object to be refined, it is preferable that the purity of the electrolytic bath be increased. From such a viewpoint, as the purity of the electrolytic bath, preferable is not less than 3 N, more preferable is not less than 5 N, and particularly preferable is not less than 6 N. Moreover, particularly, in the case where silicon or germanium is an object to be refined, the contents of P and B each are preferably not more than 0.5 ppm, more preferably not more than 0.3 ppm, and particularly preferably not more than 0.1 ppm based on the electrolytic bath.

In the present invention, alkali metal elements and alkaline earth metal elements do not need to be considered as the impurities of the electrolytic bath. In the electrolysis step, these elements are harder to move to the cathode than silicon and germanium, and are hardly mixed in the alloy of the cathode. Moreover, the metal used as the cathode medium metal does not also need to be considered as the impurities of the electrolytic bath.

In the case where a solid anode is used, according to the specific gravity of the alloy of the cathode and that of the electrolytic bath, of the cathode and the electrolytic bath, the one with a higher specific gravity can be disposed in a relatively lower position than the one with a lower specific gravity, and the anode can be disposed at a place away from the cathode, or in the electrolytic bath, the anode and the cathode can be disposed at places away from each other in the horizontal direction. Moreover, in the case where a liquid anode is used, in the electrolytic bath, the anode and the cathode can be disposed spaced from each other in the horizontal direction; or the same disposition as in the case of an aluminum three-layer electro-refining process, namely, according to the specific gravities of the liquids of the anode, the electrolytic bath, and the cathode, in the order of the cathode, the electrolytic bath, and the anode from the top or the opposite order of the anode, the electrolytic bath, and the cathode from the top, these three elements can be disposed such that the specific gravity of the element is higher at a lower position. From the viewpoint of improvement in operationability, reduction in the size of the reaction container, and uniform current distribution, the same disposition as in the case of the aluminum three-layer electro-refining process is preferable; in the case where refinement of silicon is aimed, particularly preferable is that the cathode, the electrolytic bath, and the anode are disposed in this order from the top. In the present invention, the anode and the cathode are disposed spaced from each other within the container for the electrolysis, and the anode and the cathode act through the electrolytic bath in the electrolysis step.

(Electrolysis Condition)

The electrolysis temperature is set according to the composition of the alloy of the cathode such that the alloy of the cathode is kept in a liquid phase. It is preferable that the electrolysis temperature be higher than the complete solidification temperature of the alloy of the cathode. At a higher electrolysis temperature, the solubility of the element to be refined in the alloy of the cathode is improved; accordingly, a larger amount of the element to be refined can be moved to the cathode and recovered. It is preferable that the electrolysis temperature be lower than the melting point of the element to be refined. If the electrolysis temperature is less than the melting point of the element to be refined, the current efficiency in the electrolysis is improved, and selection of the material for the container for the electrolysis is easier.

For example, in the case where the element to be refined is silicon and aluminum-silicon is used as the alloy of the cathode, the complete solidification temperature of the alloy, i.e., the eutectic point is 577° C.; accordingly, it is preferable that the electrolysis temperature be set at a temperature higher than 577° C. and lower than 1410° C. that is the melting point of silicon. For example, in the case where the element to be refined is germanium and zinc-germanium is used as the alloy of the cathode, the complete solidification temperature of the alloy, i.e., the eutectic point is 398° C.; accordingly, it is preferable that the electrolysis temperature be set at a temperature higher than 398° C. and lower than 958° C. that is the melting point of germanium.

From the viewpoint of the yield of the element to be refined during the electrolysis reaction, it is preferable that at a temperature of not more than the melting point of the element to be refined the electrolysis temperature be as high as possible.

In the case where the element to be refined is silicon, the electrolysis temperature is preferably not less than 700° C., more preferably not less than 900° C., and particularly preferably not less than 1100° C. Considering restrictions on the material for the container for the electrolysis or the like, it is preferable that the electrolysis temperature be not more than 1300° C.

In the case where the element to be refined is germanium, the electrolysis temperature is preferably not less than 500° C., more preferably not less than 600° C., and particularly preferably not less than 700° C. Considering restrictions on the material for the container for the electrolysis or the like, it is preferable that the electrolysis temperature be not more than 900° C.

For example, in the case where the element to be refined is silicon, aluminum is used as the cathode medium metal, pure aluminum is used as the cathode, and the electrolysis step is started, the melting point of aluminum is 660° C. and the eutectic point of Al—Si is approximately 580° C.; accordingly, first, the electrolysis reaction is started at a temperature not less than 660° C. at which aluminum as the cathode is a liquid phase. Then, silicon moves to the cathode to produce Al—Si on the cathode as the electrolysis progresses; after that, the electrolysis temperature can be reduced to 580° C. because the alloy can be a liquid phase at a temperature of not less than the eutectic point. However, at a temperature lower than the eutectic point, the solid is deposited to cause dendritic growth of silicon.

For example, in the case where the element to be refined is germanium, zinc is used as the cathode medium metal, pure zinc is used as the cathode, and the electrolysis step is started, the melting point of zinc is 419° C. and the eutectic point of Zn—Ge is approximately 398° C.; accordingly, first, the electrolysis reaction is started at a temperature of not less than 419° C. at which zinc as the cathode is a liquid phase. Then, germanium moves to the cathode to produce Zn—Ge on the cathode as the electrolysis progresses; after that, the electrolysis temperature can be reduced to 398° C. because the alloy can be a liquid phase at a temperature of not less than the eutectic point. However, at a temperature lower than the eutectic point, the solid is deposited to cause dendritic growth of germanium.

The atmosphere in the electrolysis step is not particularly limited; the air or an inert gas is preferable, and it is more preferable for progression of the electrolysis that no water, oxygen, and the like exist.

(Container for Electrolysis)

The material for the container for the electrolysis that accommodates the electrolytic bath is not particularly limited; those that do not react with the element to be refined and the electrolytic bath are preferable; examples thereof include oxides, nitrides, carbides, and carbonaceous materials. In the case where the element to be refined is silicon, examples of oxides include silica, alumina, zirconia, titania, zinc oxide, magnesia, and tin oxide; examples of nitrides include silicon nitride and aluminum nitride, and also include those in which these constituting elements are partially substituted by other element. For example, a compound such as sialon comprising silicon, aluminum, oxygen, and nitrogen can be used. Examples of carbides include SiC, and examples of carbonaceous materials include graphite; those in which these constituting elements are partially substituted by other element can also be used. Further, similarly to the aluminum electrolysis, a method in which the electrolytic bath is held by a solidified electrolyte (for example, cryolite) may be used.

The same holds true for the case where the element to be refined is germanium.

(Withdrawal Step)

In the withdrawal step, a part or the whole of the alloy of the cathode subjected to the electrolysis as described above is withdrawn to the outside of the container for the electrolysis. The withdrawal process is not particularly limited; withdrawal may be performed in batch or continuously.

(Deposition Step)

In the deposition step, the alloy of the cathode withdrawn from the container for the electrolysis is cooled at a temperature higher than the complete solidification temperature and lower than the electrolysis temperature to deposit the element to be refined contained in the withdrawn alloy as a solid.

If the cooling temperature is not higher than the complete solidification temperature of the alloy of the cathode, the medium metal other than the element to be refined, namely, the cathode medium metal is also deposited with the element to be refined; accordingly, it is difficult to selectively recover only the target element to be refined from the alloy of the cathode. Contrary to this, in the present invention, the cooling temperature is higher than the complete solidification temperature of the alloy of the cathode, and the concentration of the element to be refined in the alloy of the cathode withdrawn from the container for the electrolysis is higher than that of the composition corresponding to the complete solidification temperature of the alloy; accordingly, by cooling at the cooling temperature lower than the electrolysis temperature and higher than the complete solidification temperature, the element to be refined can be selectively deposited from the alloy of the cathode.

The upper limit of the cooling temperature is the electrolysis temperature. The recoverable amount of the element to be refined corresponds to the difference between the compositions in the liquidus of the alloy corresponding to the difference between the electrolysis temperature and the cooling temperature. Accordingly, in order to increase the recoverable amount of the element to be refined, it is preferable that the difference between the electrolysis temperature and the cooling temperature be larger.

In both the case where the element to be refined is silicon and the case where the element to be refined is germanium, the difference between the electrolysis temperature and the cooling temperature is preferably not less than 100° C., more preferably not less than 200° C., and still more preferably not less than 300° C. However, a larger difference in the temperature leads to a larger thermal loss; for this reason, in the case where the change of the composition in the liquidus corresponding to the temperature is sufficient, the difference between the electrolysis temperature and the cooling temperature may not be large, and cooling can be performed with an economically optimal difference of the temperature in an available range of the temperature.

The cooling temperature can be reduced to a temperature in the vicinity of the complete solidification temperature of the alloy of the cathode (for example, eutectic point); however, it is preferable from the viewpoint of easy cooling operation that the cooling temperature be not less than the melting point of the cathode medium metal.

For example, in the case where the element to be refined is silicon and an aluminum-silicon alloy is used as the alloy of the cathode, at an electrolysis temperature of 1100° C., the cathode can keep the state of the liquid phase until the concentration of silicon in the alloy of the cathode reaches approximately 55% by mass at the maximum. When the alloy is withdrawn to the outside of the container for the electrolysis and cooled to 600° C., the concentration of silicon does not become equilibrium unless the concentration of silicon is reduced to 15% by mass; accordingly, silicon corresponding to the difference of 40% by mass can be recovered as a solid.

For example, in the case where the element to be refined is germanium and a zinc-germanium alloy is used as the alloy of the cathode, at an electrolysis temperature of 800° C., the cathode can keep the state of the liquid phase until the concentration of germanium in the alloy of the cathode reaches 60% by mass at the maximum. When the alloy is withdrawn to the outside of the container for the electrolysis and cooled to 450° C., the concentration of germanium does not become equilibrium unless the concentration of germanium is reduced to 12% by mass; accordingly, germanium corresponding to the difference of 48% by mass can be recovered as a solid.

As the process for cooling the alloy of the cathode, a known process can be used. Namely, examples of the process include a process of keeping an withdrawn alloy of a cathode in a container kept at a cooling temperature; and a process of keeping an withdrawn cathode in a container kept at a temperature slightly higher than a cooling temperature, immersing a cooled body in which the cooling temperature is set in the alloy of the cathode, and depositing an element to be refined on the surface of the cooled body.

(Recovery Step)

In the recovery step, a solid deposit of the element to be refined is recovered from the alloy of the cathode cooled in the deposition step. The recovering process is not particularly limited, and examples thereof include filtration and centrifugation.

(Reuse Step)

In the reuse step, a residue, after the element to be refined deposited from the alloy of the cathode is recovered in the recovery step, is used as the cathode in the electrolysis step.

According to such a process for producing a refined metal element an element to be refined, the element to be refined and an element less noble than this are dissolved from the material for the anode to the electrolytic bath, and the element to be refined selectively moves from the electrolytic bath to the alloy of the cathode and accumulates.

The alloy of the cathode in which the concentration of the element to be refined is increased and that is a liquid phase is withdrawn from the container for the electrolysis; in the deposition step, the element to be refined is selectively deposited with high purity; in the recovery step, the element to be refined deposited from the alloy of the cathode is recovered.

In the electrolysis step, the alloy comprising the element to be refined and the medium metal and having a complete solidification temperature lower than the melting point of the element to be refined is used as the cathode; accordingly, the cathode can easily become a liquid phase, and the electrolysis temperature can be lower than that in the case where a single element to be refined is used as a cathode in a liquid phase. For this reason, the electrolysis can be performed at a temperature lower than the melting point of the element to be refined, and energy load and load on the material for the container for the electrolysis can be reduced compared to the case where a single element to be refined is used as a cathode, and it is advantageous. Moreover, because the cathode is a liquid phase, a stable electrode interface is formed; when an alloy in which the concentration of the element to be refined is higher than that in the composition corresponding to the complete solidification temperature is obtained on the cathode, dendritic growth of the element to be refined to cause a short circuit between electrodes is suppressed, and trapping of the electrolytic bath in the product of the element to be refined is suppressed.

Because the medium metal does not substantially form a solid solution with the element to be refined, the concentration of the element to be refined in the alloy of the cathode subjected to the electrolysis step and the withdrawal step and brought to the deposition step is higher than that of the composition corresponding to the complete solidification temperature; thereby, the element to be refined contained in the cathode can be selectively deposited with high purity by cooling. Thereby, the purity of the recovered metal element or metalloid element can be sufficiently higher than that of the material that forms the anode, and the refined element to be refined can be easily obtained.

In the reuse step, the residue, after the element to be refined deposited from the cooled alloy of the cathode is recovered, is returned to the cathode in the electrolysis step; thereby, the alloy in which the concentration of the element to be refined is sufficiently reduced can be reused as the cathode, and movement of the element to be refined from the material for the anode to the cathode and increase in the concentration can be continuously performed with high efficiency. Accordingly, in the electrolysis step, the element to be refined does not reach a saturated concentration to cause stagnation of the electrolysis, and refinement can be continuously performed as long as the fluidity of the alloy of the cathode can be maintained.

Moreover, it is preferable that the anode be an alloy that is a liquid phase at the electrolysis temperature; thereby, the material serving as the anode is easily added to the electrolytic bath properly, and the electrolysis step is more easily continuously operated.

In the case where the element to be refined is silicon, according to the present invention, more than 40% by mass of silicon can be obtained in the electrolysis step based on the cathode (including the case where aluminum is used as the cathode medium metal and pure aluminum is used as the cathode at the time of starting the electrolysis step), and for example, not less than 45% by mass of silicon can be obtained in the recovery step. Moreover, in the case where the element to be refined is germanium, more than 40% by mass of germanium can be obtained in the electrolysis step based on the cathode (including the case where zinc is used as the cathode medium metal and pure zinc is used as the cathode at the time of starting the electrolysis step), and for example, more than 50% by mass of germanium can be obtained. Accordingly, according to the present invention, the yields of the obtained silicon and germanium are particularly high, and it is economically advantageous. In the process according to the present invention, the amount to be produced is controlled by the current.

The purity of the thus-obtained recovered product of the element to be refined is extremely higher than that of the material for the anode as the raw material, and the recovered product is suitably used for electronic devices and spattering targets, and a raw material for silicon for solar cells particularly in the case of silicon.

When necessary, the obtained recovered product of the element to be refined is treated with an acid or an alkali in order to remove an adhering residue of a metal component and a non-reacted metal component, and further segregation such as directional solidification, dissolution under high vacuum, and the like are performed; thereby, impurity elements contained in the recovered product of the element to be refined can be further reduced; in the case of silicon, it is particularly preferable that the obtained polycrystalline silicon be subjected to directional solidification to increase the purity.

In the present invention, silicon obtained in the case where the element to be refined is silicon, for example, a solar cell using polycrystalline silicon will be described.

Using silicon obtained according to the present invention, an ingot is produced by a casting process or an electromagnetic casting process. The conductivity type of a substrate in the solar cell is usually a p type. For example, boron is added to silicon or aluminum is left at the time of refining silicon; thereby, a dopant can be introduced into silicon to obtain a p type silicon. The ingot is sliced using an inner diameter blade cutting, a multi-wire saw or the like. After slicing, when necessary, the sliced surfaces are wrapped using free abrasive grains; further, in order to remove the damaged layer, the wrapped ingot slice is e.g., immersed in an etching solution such as hydrofluoric acid to obtain a polycrystalline silicon substrate. In order to reduce optical reflection loss on the surface of the polycrystalline silicon substrate, a V-groove is mechanically formed on the surface using a dicing machine, or a texture structure is formed by reactive ion etching or etching using an acid, an alkali, or the like. Subsequently, a diffusion layer with an n type dopant such as phosphorus and arsenic is formed on the light receiving surface of the polycrystalline silicon substrate to obtain a p-n junction. Further, a film layer of an oxide of TiO₂ or the like is formed on the surface of the diffusion layer, electrodes are provided in the respective layers, and an antireflection film formed with MgF₂ or the like in order to reduce loss of optical energy caused by reflection is provided; thus, a solar cell can be produced.

In the description above, the embodiment according to the present invention has been described, but the embodiment according to the present invention disclosed above is only an example, and the scope of the present invention will not be limited to the embodiment. The scope of the present invention is specified by the claims of the patent, and includes all modifications equivalent to the description in the claims of the patent in the meaning and the scope thereof.

EXAMPLES

Examples will be shown in order to describe the present invention more in detail, but the present invention will not be limited to these.

Example 1

Aluminum, cryolite, and silica are placed into a graphite crucible, and the crucible is set within an electric furnace having a mullite furnace tube. Next, solid silicon containing impurities is immersed in an electrolytic bath at 1100° C., and electrolysis is performed using the solid silicon as the anode and aluminum in a liquid phase provided on the bottom of the graphite crucible as the cathode.

After the electrolysis, the alloy of the cathode is recovered by cooling. The obtained alloy is dissolved by hydrochloric acid; thereby, refined silicon can be obtained. Moreover, the alloy of the cathode after the electrolysis is withdrawn while at 1100° C., and kept at 700° C. for 3 hours to be partially deposited for solid-liquid separation; thereby, a deposit in which the concentration of silicon is relatively high and a liquid alloy in which the concentration of silicon is relatively low are obtained, and refined silicon can be obtained. The liquid alloy (residue) after the deposit (refined silicon) is separated can be returned to the graphite crucible in the electrolysis furnace again to perform electro-refining of silicon.

Example 2

An alloy of copper and silicon, cryolite, silica, calcium chloride, barium chloride, and an alloy of aluminum and silicon are placed into a magnesia crucible, and the crucible is set within an electric furnace having a mullite furnace tube. Next, electrolysis is performed at 1100° C. using the alloy of copper and silicon as the anode and the alloy of aluminum and silicon as the cathode.

After the electrolysis, the alloy of the cathode is recovered by cooling. The obtained alloy of the cathode is dissolved by hydrochloric acid; thereby, refined silicon can be obtained. Moreover, the alloy after the electrolysis is withdrawn while at 1100° C., and kept at 700° C. for 3 hours to be partially deposited for solid-liquid separation; thereby, a deposit in which the concentration of silicon is relatively high and a liquid alloy in which the concentration of silicon is relatively low are obtained, and refined silicon can be obtained. The liquid alloy (residue) after the deposit (refined silicon) is separated can be returned to the magnesia crucible in the electrolysis furnace again to perform electro-refining of silicon. 

1. A process for producing a refined metal or metalloid, comprising: an electrolysis step of, in an electrolytic bath provided within an electrolyzer where a material comprising a metal element or metalloid element and impurities is subjected to act as an anode, and an alloy comprising the same metal element or metalloid element as the metal element or metalloid element contained in the anode and a solvent metal that does not substantially form a solid solution with the metal element or metalloid element and having a complete solidification temperature lower than a melting point of the metal element or metalloid element is subjected to act as a cathode, performing electrolysis at an electrolysis temperature allowing the alloy to be a liquid phase to move the metal element or metalloid element in the anode into the alloy of the cathode; an withdrawal step of withdrawing part or a whole of the alloy of the cathode to an outside of the electrolyzer after the electrolysis step; a deposition step of cooling the withdrawn alloy at a temperature higher than the complete solidification temperature and lower than the electrolysis temperature to deposit the metal element or metalloid element contained in the alloy; and a recovery step of recovering the metal element or metalloid element deposited from the cooled alloy.
 2. The process according to claim 1, wherein the solvent metal has a eutectic point with the metal element or metalloid element.
 3. The process according to claim 1, further comprising a reuse step of using a residue after the metal element or metalloid element deposited is recovered from the cooled alloy, as the cathode in the electrolysis step.
 4. The process according claim 1, wherein the metal element or metalloid element is silicon or germanium.
 5. The process according to claim 4, wherein the alloy of the cathode comprises one or more metal elements selected from the group consisting of aluminum, silver, copper, and zinc.
 6. The process according to claim 4, wherein the material for the anode comprises one or more metal elements selected from the group consisting of silver, copper, tin, and lead.
 7. The process according to claim 1, wherein the electrolytic bath comprises cryolite.
 8. The process according to claim 1, wherein the electrolysis temperature is higher than the complete solidification temperature and lower than the melting point of the metal element or metalloid element. 